1. Field of the Invention
The present invention relates to noise reduction in a transmitter and, more particularly, to noise reduction in a transmitter in a cellular base transceiver station using a direct up-conversion scheme.
2. Description of Related Art
In a wireless telephony network several base transceiver stations are connected to a central switching office which serves as an interface between the base transceiver stations and a public wired telephone network or other wireless telephone switching offices. The base transceiver stations also communicate with mobile telephones or a multiscriber telephone system (such as an office or hotel) over selected radio frequency channels.
Some prior art transmitters in base transceiver stations include quadrature modulators, which are well known and have been widely used in digital and some analog modulation schemes. A typical prior art quadrature modulator 10 is shown in FIG. 1A signal having an in-phase component (I) and a quadrature phase component (Q) is input to the quadrature modulator 10. A first mixer 12 mixes an output of a local oscillator 14, operating at a center frequency .omega..sub.c, in phase with the I signal components resulting in a signal Icos(.omega..sub.c t). A second mixer 16 mixes the output of the local oscillator 14, which is phase shifted by 90.degree. by a phase shifter 18, with the Q signal component, resulting in a signal Qsin(.omega..sub.c t). A summer 20 then combines the two signals, resulting in a quadrature modulated signal Icos(.omega..sub.c t)+Qsin(.omega..sub.c t).
In a wireless telephony network, the frequency band is divided in 30 KHz channels. Wireless transmissions over a selected channel must comply with the Telecommunications Industry Association's (TIA) IS-54 specification, including emissions specification 3.4.1.1.B-(2) which states that the emission of a transmitter must be 80 dBc or lower than an unmodulated carrier at .+-.45 KHz or greater offset from the intermediate frequency in a 300 Hz bandwidth In order to meet this IS-54 specification, prior art transmitters in base transceiver stations implemented dual (or even triple) conversion techniques to filter out noise at the intermediate frequency. Such prior art systems used expensive surface acoustic wave (SAW) or crystal filters for noise filtering. Other prior art systems which implemented direct up-conversion techniques required expensive and bulky radio frequency filters to filter out noise.
FIG. 2 is a block diagram of a prior art dual conversion transmitter 22 using a SAW filter for noise reduction. Digitally modulated I and Q components of an input signal are provided to digital to analog (D/A) converters 24 and 26, respectively, where they are converted to analog signal components. The I and Q analog signal components are then provided to a first converter 28, which up-converts the signals to an intermediate frequency.
The first converter 28 comprises a quadrature modulator, as described in connection with FIG. 1, wherein a first mixer 30 mixes the I input signal component in-phase with an output of a local oscillator 32. A second mixer 34 and a phase shifter 36 mix the Q input signal component 90.degree. out of phase with the output of the local oscillator 32. The outputs of the first and second mixers 30 and 34 are combined by a summer 38. The modulated signal output from the summer 38 is then passed through a bandpass SAW filter 40 with a center frequency equal to the intermediate frequency. The bandpass SAW filter 40 filters the noise at all frequencies outside of .+-.45 KHz of the intermediate frequency. Often, more than one SAW filter is required to adequately filter out noise from the signal.
The output from the SAW filter 40 is then provided to a second converter 42, wherein a third mixer 44 combines the signal with the output of a synthesizer 46 to convert the signal to a radio frequency within a transmit band or channel. The radio frequency signal is then passed through a band pass filter 48, which filters out any spurious signals outside of the transmit channel. The resultant signal may then be amplified and transmitted to, for example, a mobile phone or a multisubscriber telephone system.
The prior art dual conversion system of FIG. 2 is complicated and expensive to implement because of the use of the SAW filter 40 and the need for the two separate converters 28 and 42. Thus, there remains a need for a simple, inexpensive system that allows for a direct up-conversion scheme in a transmitter that meets the stringent IS-54 emission requirements.